Technical Field
The present invention relates to an optical transmitter that can be used as an optical transmission section of a subscriber side device, and to a light source temperature change suppression method in which the optical transmitter is provided.
Related Art
A communications network that links a building (a station) belonging to a communications business with subscriber homes is referred to as an access network. Recently, optical access networks that can handle increasing communication volumes and transfer huge amounts of information across access networks by using optical communications have been becoming common.
One type of optical access network is the passive optical network (PON). A PON has a configuration that is equipped with an optical splitter between a single station side device (optical line terminal (OLT)) provided at a station and plural subscriber side devices (optical network units (ONUs)) provided at respective subscriber homes. The OLT and ONUs are connected with the optical splitter by optical fibers.
A single-core optical fiber is used for the connection between the OLT and the optical splitter. This single-core optical fiber is split by the optical splitter and shared by the plural ONUs. Optical splitters are cheap, passive components. Thus, PONs are economically excellent and easy to maintain. Accordingly, the introduction of PONs is proceeding rapidly.
In a PON, signals sent from ONUs to the OLT (which may hereinafter be referred to as “upstream optical signals”) are combined by the optical splitter and transmitted to the OLT. Meanwhile, signals sent from the OLT to the ONUs (which may hereinafter be referred to as “downstream optical signals”) are separated by the optical splitter and transmitted to the respective ONUs. In order to prevent interference between the upstream optical signals and the downstream optical signals, respectively different wavelengths are assigned to the upstream optical signals and the downstream optical signals.
Various multiplexing technologies are employed in PONs. The multiplexing technologies employed in PONs include time division multiplexing (TDM) technologies in which short time slots on the time axis are assigned to respective subscribers, wavelength division multiplexing (WDM) technologies in which different wavelengths are assigned to respective subscribers, code division multiplexing (CDM) technologies in which different codes are assigned to respective subscribers, and so forth.
Of these multiplexing technologies, TDM is employed in TDM-PONs, which are currently particularly widely used. In a TDM-PON, time division multiple access (TDMA) is used. TDMA is a technology in which the OLT administers transmission timings of the ONUs and performs control such that upstream optical signals from different ONUs do not collide. The ONUs use assigned time slots on the time axis, with transmission timings specified by the OLT, and transmit upstream optical signals in bursts (burst signals).
Among PON systems, systems that employ Ethernet (registered trademark) technology are referred to as Ethernet-PON, and systems that employ Gigabit (1×109 bits/s) Ethernet (registered trademark) are referred to as GE-PON. A GE-PON conforms to, for example, IEEE 802.3ah, IEEE 802.3av or the like.
A PON that combines TDM and WDM (which may hereinafter be referred to as a TDM/WDM-PON) has been proposed (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 2011-82908 (Patent Reference 1) and 2011-135280 (Patent Reference 2)). In an example of a TDM/WDM-PON, the OLT includes a plural number of terminal devices (optical subscriber units (OSU)).
In the TDM/WDM-PON, respectively different transmission wavelengths are assigned to the OSUs. The OSUs transmit downstream optical signals with the assigned transmission wavelengths. Meanwhile, the ONUs transmit upstream optical signals at transmission wavelengths and transmission timings notified to the ONUs by downstream optical signals from the OSUs with which the ONUs are registered.
In the TDM/WDM-PON, because communications are performed at wavelengths corresponding to the OSUs, the wavelengths at which ONUs transmit and receive are variable. Therefore, a reception section of each ONU is provided with a wavelength-variable filter. Thus, a reception wavelength for downstream optical signals is variable. In addition, a transmission section of each ONU is provided with an optical transmitter in which the wavelength is variable. Thus, the wavelength of upstream optical signals to be transmitted is variable.
It is sufficient that an ONU be registered with any of the plural OSUs. Therefore, in a TDM/WDM-PON, the ONUs to be administered may be divided up between and administered by the plural OSUs. An OLT may manage the numbers of ONUs corresponding with the respective OSUs in accordance with communication conditions (for example, traffic conditions and suchlike) by performing dynamic wavelength allocation (DWA).
Because of these features, in a TDM/WDM-PON, a larger number of ONUs may be accommodated by an OLT than heretofore. In a TDM/WDM-PON accommodating a large number of ONUs, in order to avoid collisions between the upstream optical signals transmitted by the respective ONUs (which are burst signals), the burst signals must be generated with a large extinction ratio (General Conference of the Institute of Electronics, Information and Communication Engineers, March 2013, B-8-41, Iwamura et al. (Non-patent Reference 1)).
As optical transmitters that generate burst signals with large extinction ratios, optical transmitters equipped with an optical amplifier such as, for example, a semiconductor optical amplifier (SOA) or the like have been proposed (Society Conference of the Institute of Electronics, Information and Communication Engineers, September 2013, B-8-26, Iwamura et al. (Non-patent Reference 2)). In an SOA, there is a large difference in light output level between On-operation times and Off-operation times. An optical transmitter according to Non-patent Reference 2 generates burst signals with a large extinction ratio by utilizing this characteristic of an SOA.
As mentioned above, an optical transmitter provided at an ONU in a TDM/WDM-PON is configured with a variable transmission wavelength. As a structure that can vary a transmission wavelength, a structure is known that employs a laser diode (LD) array as a light source of the optical transmitter. An LD array includes plural LDs specified with respectively different wavelengths. The optical transmitter selects an LD with a wavelength corresponding to an OSU that is administering the ONU in response to a command from an ONU-MAC (media access control). In addition, the LD array can be temperature-adjusted by a thermo-electric cooler (TEC). Thus, the wavelength of the upstream optical signals is determined by which LD is selected and by temperature adjustment by the TEC.
Output light from the LD array is a continuous wave (CW). Burst signals are generated from the output light of the LD array by an optical amplifier such as the above-mentioned SOA or the like.
The SOA turns on at a burst signal generation timing commanded by the ONU-MAC and amplifies the output light of the LD array. The SOA turns off after a period corresponding to the duration of the burst signal that is to be generated. Thus, the burst signals are generated in bursts from the output light of the LD array by on/off operation of the SOA. Burst signal generation timings commanded by the ONU-MAC correspond with burst signal transmission timings designated by the OSU. The burst signals generated by the SOA are modulated with data at a modulator and transmitted to the OSU.
The wavelength of the burst signals outputted from the optical transmitter is monitored by a wavelength monitor. On the basis of monitoring results, the wavelength of the burst signals may be corrected by temperature adjustments by the TEC and may be kept within a certain range.
When an optical transmitter is installed in an ONU, it is preferable if the respective structural elements of the optical transmitter are integrated onto a single chip, with a view to lowering costs, reducing power consumption, reducing size of the ONU and the like. Examples of an optical transmitter integrated onto a single chip include an optical transmitter in which an LD array, an optical coupler, an SOA and so forth structuring an optical transmitter are integrated onto a 500 μm×2600 μm chip (bulletin from Furukawa Electric Co., Ltd., volume 121, March 2008 (Non-patent Reference 3)).
During On operations, an SOA heats up. Therefore, the temperature of the SOA rises and falls in association with On/Off operations.
In an optical transmitter that is integrated on a chip, the distance between an LD array and an SOA is short. Therefore, rises and falls in the temperature of the SOA affect the LD array. As a result, temperature changes occur at the LD array. As mentioned above, the SOA performs On/Off operations in accordance with burst signal generation timings. Therefore, temperature rises and falls of the SOA occur at the burst signal generation timings. Hence, temperature changes also occur at the LD array at the burst signal generation timings. These temperature changes at the LD array cause wavelength fluctuations in the burst signals that are transmitted. If wavelength fluctuations occur in burst signals in a TDM/WDM-PON in which the transmission wavelengths of ONUs are specified in accordance with the OSUs administering the ONUs, stability of the system deteriorates. For example, if a spacing between the transmission wavelengths of respective ONUs specified by the respective OSUs administering the ONUs is 0.8 nm, then wavelength fluctuations of the burst signals must be kept to less than 0.8 nm.
The duration of a burst signal is short, being tens of microseconds. In contrast, a duration of several seconds is required for temperature adjustment by a TEC. Therefore, if temperature adjustment by a TEC starts after a temperature change caused by the generation of a burst signal has occurred, the temperature adjustment cannot correct wavelength fluctuations in the burst signal.